This invention relates generally to the field of seismic data processing. Specifically, the invention is a method of calculating a throw volume for quantitative fault analysis.
Fault interpretation plays an important role in the understanding of the segmentation of hydrocarbon reservoirs. Evaluating the risk for fault seals in reservoirs requires knowledge of three parameters: strike, dip and throw along the fault. At present, these parameters are generally extracted from 2D depth structure maps generated from fault and horizon interpretations.
Faults may be detected by looking at vertical offsets of seismic reflectors in a volume of seismic data. As will be understood to those skilled in the art, the continuity of seismic reflectors in a volume of seismic amplitude data may be quantified by computing the correlation coefficient between adjacent seismic traces over a movable vertical window. A low coefficient of correlation indicates that the reflector is discontinuous at the location of those traces. The three dimensional representation of calculated correlation coefficients which results thereby provides an analytical, and in graphical form a visual, representation of the amount of discontinuity in the reflectors in the seismic data volume. Such a volume is called a discontinuity volume. However, discontinuity analysis does not provide information on the amount of throw across faults, and does not provide a mechanism for determining dip direction when scrolling through time slices of a volume of seismic data.
Several techniques have been used in the oil industry to generate time lag volumes, also called dip volumes, for use in steering coherency calculations along dipping reflectors. Many of these techniques are based on cross correlation analyses that compute the time lag, which is also referred to as the time dip, necessary to flatten the reflectors. However, because time lag associated with reflector flattening may be independent of the time lag associated with fault throw, these techniques are not tuned to obtain the correct lag associated with a fault. Furthermore, it is not necessarily desirable to align or flatten reflectors across faults if the goal of an analysis is to image discontinuities. In addition, for best results in dip-steering discontinuity imaging, the time lag obtained from cross-correlation calculations should be measured at a regional scale and not just locally, as is the case with many of the time lag volume-related techniques.
More specifically, prior art techniques using cross-correlation-based analyses for discontinuity disclose methods that begin with an estimate that first applies a pre-defined dip azimuth measurement axis to remove a significant portion of the regional structural dip. Next a semblance calculation is performed, as a function of time to multiple seismic traces, to further estimate and correct for local dip. During this step a maximum semblance cube may be created that highlights stratigraphic and structural discontinuities, corrected for structural dips. Such techniques have as a principal objective the production of cross-correlation, semblance, or coherence measures of reflector terminations, such as faults or channel edges. A by-product of the computations is a time-dip or lag volume which describes a smooth, apparent, dip of reflectors but at the same time, and directly as a result of the effect of smoothing, contains no information about throw across faults.
Another technique applied in the art examines the similarity of multiple traces at various time lags to estimate the dip of reflectors. An eigenstructure algorithm is then used to calculate the similarity of traces in the locally averaged dip direction. In this technique, the best discontinuity images, in other words those most useful for interpretation purposes, are generated by smoothing locally estimated dip measurements to obtain the regional dip and using the smoothed regional dip to steer the coherency calculation. The technique allows the attenuated effect of the local dip measurement across faults to result in better images of fault discontinuity. Although a dip cube is a by-product of the computation, throw information is not obtained because of the smoothing that is applied.
Although information may be gained in fault throw measurements from structural maps generated from manual fault and horizon interpretation, cost issues and trade-offs in cycle-time, potential subjectivity, and density of observations arise.
None of the above-described techniques make use of the lag volume for imaging faults or obtaining throw information along faults. Thus there exists a need for a method to generate fault orientations and fault throws throughout a volume of seismic amplitude data for quantitative evaluation of fault parameters that can be used to facilitate fault interpretation and also to evaluate fault seal potential.
The invention is a method for analysing faults in a three-dimensional volume of seismic data by calculating a throw volume. A range of time shifts and a search direction for the seismic data volume are selected. A data location separation and a vertical time window are also selected. A cross-correlation is calculated between data values corresponding to first and second data locations separated by the data location separation and symmetrically located in the search direction on each side of a target data location. The cross-correlation is calculated throughout the vertical time window for each time shift in the range of time shifts. The time shift corresponding to the maximum calculated cross-correlation is stored in the throw volume.